1. Field of the Invention
The present invention relates to an optical disk apparatus that uses laser beams to retrieve digital data stored in an optical data storage medium. The disk apparatus may also be designed to write data to an optical disk only once or any number of times by magnetic field modulation or optical strength modulation for example.
2. Description of the Related Art
FIGS. 11 and 12 of the accompanying drawings show two different types of conventional optical disk apparatus provided with a slider 80 or 90, respectively. The disk apparatus 8 of FIG. 11 is a linear arm type including a carriage 81 for supporting the slider 80, an objective lens system 82 and a tracking mechanism 83. The disk apparatus 9 of FIG. 12 is a swing arm type including a carriage 91 for supporting the slider 90, an objective lens system 92 and a tracking mechanism 93.
To attain high numerical aperture (NA), each of the lens systems 82 and 92 is composed of two lenses 82a˜82b or 92a˜92b. Each of the tracking mechanisms 83 and 93 is designed to enable minute shifting of the beam spot in the radial direction of the optical disk D (see the double-headed arrow A-B) so that the beam spot is to be located on the required track of the storage disk.
In the disk apparatus 8 of FIG. 11, the first lens 82a is supported by the carriage 81 via the tracking mechanism 83, while the second lens 82b is held by the slider 80. To perform the tracking control, the first lens 82a is moved in the radial direction of the disk D by the tracking mechanism 83.
The first lens 82a is larger than the second lens 82b so that it can properly receive the light from the light source. Due to the larger size, the first lens 82a is often heavier than the second lens 82b. 
In the disk apparatus 8 of FIG. 11, the tracking mechanism 83 needs to have a heavy and complicated structure to move the relatively heavy lens 82a in the tracking direction. Unfavorably, such a mechanism tends to be expensive and consume much power during a data-processing operation. In addition, since the first lens 82a is heavy, the tracking movement of the lens 82a may not be quick enough, which is disadvantageous to achieving high-speed recording. Further, the overall thickness of the carriage 81 may unduly be large, since the first lens 82a and the tracking mechanism 83 are both mounted on the carriage 81.
In the disk apparatus 9 of FIG. 12, the first lens 92a and the second lens 92b (which constitute the lens system 92) are held by the slider 90. The tracking mechanism 93 includes a galvano-mirror supported by the swing arm 91. The light emitted from the light source 94a of an optical unit 94 is reflected on the galvano-mirror 93 and another mirror 95, to be directed upward. Then, the light passes through the lens system 92 to make a beam spot on the disk D.
In the above design, the deflection surface of the galvano-mirror 93 is minutely moved for performing tracking control, so that the beam spot on the disk D is caused to shift in the radial direction of the disk. In the disk apparatus 9, the relatively heavy first lens 92a is mounted on the slider 90 together with the second lens 92b. Since the slider 90 can automatically adjust its posture and spacing with respect to the disk D, no additional device is needed on the swing arm 91 for performing the focusing control of the first lens 92a. Accordingly, the swing arm 91 is made slim. Further, in the apparatus 9, the tracking control is performed by driving the galvano-mirror 93 but not the objective lenses. Thus, the tracking operation does not need much power, and the tracking response is satisfactory. However, the galvano-mirror 93 costs much in production, and this makes the product mirror 93 expensive.